Along with the widespread use of optical communications, speeding-up of a communication rate such as from 10 Gbit/s to 25 Gbit/s, further 40 Gbit/s is advancing in a metro optical communication network for connection of relay stations between cities. In this metro optical communication, for example, in a case of 10 Gbit/s, a long-distance transmission of 40 to 80 km transmission in a single mode fiber (SMF) is required (a transmission distance to be required reduces typically in inverse proportion to the square of a bit rate (modulation rate)), and it is an important task to achieve miniaturization, a low power consumption and a low chirp in an optical transmission module.
External modulation systems low in a chirp are in general used for performing the high-speed/long distance transmission as described above. Among them, an electroabsorption (EA) modulator making use of an electroabsoprption effect has an excellent advantage in terms of miniaturization, a low power consumption, an integration performance to a semiconductor laser, and the like. Particularly a semiconductor optical integrated element (EA-DFB laser) in which an EA modulation element and a distributed feedback (DFB) laser excellent in single wavelength properties are monolithically-integrated on a single semiconductor substrate has been widely used as a light emitting device for high-speed/long distance transmission. As to signal optical wavelengths, a band of 1.5 μm small in a propagation loss of an optical fiber or a band of 1.3 μm low in a dispersion of an optical fiber is primarily used.
For driving the EA-DFB laser, injection of an electric current Iop into the DFB laser, application of a DC bias Vb to the EA modulator and application of a high-frequency bias Vpp to the EA modulator are required. When a negative voltage is applied to the DC bias Vb to increase the absolute value, a chirp value βc of modulation light is reduced, making it possible to suppress waveform degradation in the long distance transmission as well.
FIG. 1A and FIG. 1B illustrate dependence of a chirp value in regard to a relationship between an optical signal waveform and a transmission distance. FIG. 1A illustrates a relationship between an optical waveform and a transmission distance when a chirp value βc=1 and FIG. 1B illustrates a relationship between an optical waveform and a transmission distance when the chirp value βc=−0.7. As illustrated in FIG. 1A, in a case where the chirp value βc is a positive value (for example, the chirp value βc=1), the optical waveform greatly degrades after transmission of a long distance of a transmission distance 40 km or more. In contrast, as illustrated in FIG. 1B, in a case where the chirp value βc is a negative value (for example, the chirp value βc=−0.7), it is possible to suppress degradation of the optical waveform after transmission of a long distance of a transmission distance 40 km or more.
As illustrated in FIG. 1A and FIG. 1B, according to the conventional EA-DFB laser, a shape of a waveform in modulated light emitted from the EA-DFB laser degrades the more due to the chirping as the transmission distance becomes a longer distance. Therefore, for suppressing the degradation of the optical waveform in the conventional EA-DFB laser, a negative voltage is applied to the DC bias Vb to be applied to the EA modulator to increase an absolute value of the DC bias Vb and make the chirp value βc a negative value for transmission. However, when the absolute value of the DC bias Vb is made large, a loss of the EA modulator increases to cause an optical power of light to be outputted from the DFB laser to be largely lost. Therefore, in the conventional EA-DFB laser, it is difficult to acquire the optical power enough for the long distance transmission.
In this manner, the DC bias Vb to be applied to the EA modulator has a tradeoff relationship that the absolute value is the smaller, the better for acquiring a large output light power and the absolute value is the larger, the better for acquiring an optical waveform for possible long distance transmission. For overcoming this tradeoff, NPL 1 reports a method for integrating a semiconductor optical amplifier (SOA) in an output end of the EA modulator. In the configuration in the description in NPL 1, an electric current injection is performed in the SOA integrated in the output end of the EA modulator. Consequently, the positive chirp value of the modulated light outputted from the EA modulator is converted in a chirp value upon propagating in the SOA to become a negative value chirp. Therefore, it is possible to realize a state suitable for the long distance transmission.